The following invention relates to electrical transmission lines and particularly to microstrip transmission lines for making electrical connections between microprocessor-based systems in which the transmission line comprises a plurality of signal-carrying lines and two or more ground planes arrnaged on a trip of dielectric material.
Microprocessor-based systems such as computers or microprocessor emulators are often connected to each other by means of transmission lines which consist of a flat strip or ribbon of dielectric material supporting a large plurality of signal-carrying lines on one side of the ribbon and a ground-plane conductor on the other side. Typical ribbon signal-carriers of this type have high signal path denstites and may include 20 or more signal lines connecting the various I/O pins on respective microprocessor or integrated chip semiconductor elements.
With such lines it is important that the characteristic impedance Z.sub.0 be made to match the terminating impedance at the output of the transmission line. Such impedances usually range from 50 to 132 ohms. If the transmission line impedance does not match the terminating impedance of the input device, at the transmission line ouptput a portion of the electrical signal carried by the transmission line will be reflected at the interface between the line and the input. This reflection will distort or alter the signal carried by the transmission line, resulting in either attenuation of the signal amplitude or distortion of the waveform. This problem is particularly acute for high-frequency devices such as microprocessors that generate digital pulses having extremely fast rise times and operate at bandwidths in the 300-megahertz to 1-gigahertz range and higher. Reflection of the transmitted signal can alter the shape of the waveforms at these frequencies to the extent that binary information is lost.
A major component of this problem is that the characteristic impedance depends in part upon the capacitance between the signal paths and the ground plane. Typically for impedances in the 50-ohm range and for a transmission line having a high signal path densisty, the capacitance has been too high to transmit the aforementioned high frequency signals without considerable distortion of the waveform. This alters the shape of the pulse, causing its leading and trailing edges to lose definition. These pulse are intended to be square waves, but the line capacitance frequently distorsts the wave and converts it to a more rounded shape.
In the past, various geometries have been proposed for transmission lines in order to provide the necessary characteristic impedance. The most conventional type of transmission line includes a dielectric ribbon or strip of a predetermined width carrying a plurality of signal lines arranged in parallel along one side of the strip, and having a wide ground plane, substantially occupying the width of the strip, disposed on the other side. In yet another configuration, signal-carrying lines may be sandwiched between two dielectric ribbons which have ground-plane conductors disposed on the outsides of both ribbons. Such transmission line structures are shown in a text published by Motorola Semi-Conductor Products, Inc. entitled MECL System Design Handbook by William R. Blood, Jr.